So I created a bridge in Blender 2.65a, starting from a plane mesh which I extruded (as flat surfaces) along the x, y, and z axes; there are a couple of plane surfaces on diagonals, but these were created as faces, not extrusions. I find places in the completed mesh which appear as if a subsurface modifier has been applied: edges and that should be hidden are partly visible, and the solid surface does not come all the way to the face. However, no modifiers have been applied, so it's probably not a surprise that marking the edge as sharp; trying to crease and edge, using a loop cut seem to work to make the mesh look as solid as I expect to see it. However, when rendered with either the Blender render engine, and the newer Blender cycles engine, the rendered image looks as solid as I would expect.

It sounds like the normals may have gotten messed up. Go into edit mode, vertex select, all, then click the recalculate button on the left side in the tools area. You may need to scroll down to see it.

It sounds like the normals may have gotten messed up. Go into edit mode, vertex select, all, then click the recalculate button on the left side in the tools area. You may need to scroll down to see it.